1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a method of forming an alignment layer for the liquid crystal display device.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes two substrates that are spaced apart and face each other with a liquid crystal material layer interposed between the two substrates. Each of the substrates includes electrodes that face each other. Voltages applied to each of theses electrode induces an electric field between the electrodes and within the liquid crystal material layer.
The liquid crystal material layer includes a dielectric anisotropic material having spontaneous polarization properties. Accordingly, when an electric field is induced in the liquid crystal material layer, the liquid crystal molecules form a dipole due to the spontaneous polarization characteristic of the liquid crystal material. Thus, the liquid crystal molecules of the liquid crystal material layer are arranged to correspond with the direction of the applied electric field. Optical modulation of the liquid crystal material layer occurs in accordance with to the arrangement of the liquid crystal molecules. Therefore, images are produced on the LCD device by controlling light transmittance of the liquid crystal material layer using optical modulation.
FIG. 1 is an exploded perspective view of a liquid crystal display (LCD) device according to the related art. In FIG. 1, an LCD device 11 has upper and lower substrates 5 and 10, which are spaced apart from and facing each other. A liquid crystal material layer 9 is interposed between the upper and lower substrates 5 and 10.
The upper substrate 5 includes a black matrix 6, a color filter layer 7, and a transparent common electrode 18 disposed on a surface of the upper substrate 5 that faces the lower substrate 10. The black matrix 6 has openings such that the color filter layer 7 is in each of the openings of the black matrix 6. Each opening of the black matrix 6 has a color filter layer 7 including one of the three sub-color filters of red (R), green (G), and blue (B). Accordingly, the upper substrate 5 can be referred to as the color filter substrate.
A gate line 14 and a data line 22 are formed on a surface of the lower substrate 10 facing the upper substrate 5. The gate line 14 and the date line 22 cross each other to define a pixel area P. A thin film transistor T is formed adjacent to the crossing of the gate line 14 and the data line 22. The thin film transistor T has a gate electrode, a source electrode, and a drain electrode. A pixel electrode 36, which is connected to the thin film transistor T, is formed within the pixel area P and corresponds to one of the sub-color filters. The pixel electrode 36 is made of a transparent conductive material, such as indium-tin-oxide (ITO). The lower substrate 22 can be referred to as an array substrate.
In the LCD device of the related art, the array substrate is fabricated by forming a plurality of switching elements, such as thin film transistors, a plurality of pixel electrodes corresponding to the plurality of switching elements, the gate and data lines crossing each other and connected to the switching elements, and pads disposed at ends of the gate and data lines through deposition, photolithography, and etching processes. The color filter substrate is manufactured by sequentially forming the black matrix, the color filter layer and the common electrode on a surface facing the array substrate. The array substrate and the color filter substrate are then attached to each other. The LCD device is completed by injecting liquid crystal material into a space between the array substrate and the color filter substrate.
The LCD device utilizes electro-optical effect of the liquid crystal material, which are determined by anisotropies of the liquid crystal material and the arrangement of liquid crystal molecules in the liquid crystal material. Therefore, controlling the arrangement of the liquid crystal molecules has a large effect on imaging properties of an LCD device. To make an initial arrangement of the liquid crystal molecules uniform, an aligning process is performed. The aligning process may be accomplished by a rubbing method, for example. An alignment layer of a predetermined thickness may be formed on the array substrate and then hardened. In addition or in the alternative, the alignment layer can be formed on the color substrate. To arrange the surface of the alignment layer in a specified direction, the hardened alignment layer is rubbed with a particular cloth, which is referred to as a rubbing fabric. Thus, polymer chains in the alignment layer are arranged in the specified direction, so that the initial arrangement of the liquid crystal molecules may be made uniform.
The alignment layer makes the liquid crystal molecules possess the order and uniformity of single crystal. An alignment layer can be categorized as an inorganic alignment layer and an organic alignment layer. Polyimide (PI) is an organic alignment layer that is widely used.
The alignment layer may be generally formed by a roll coating method, which uses a printing apparatus including a plurality of rolls and a printing table and a transfer film attached to the printing apparatus. Formation of the alignment layer will be explained in detail with reference to FIG. 2. FIG. 2 shows a process of printing an alignment layer using a roll coating method according to the related art.
In FIG. 2, a printing apparatus 1 includes a printing table 2 that reciprocates in a plane, a printing roll 5 that rotates in correspondence with a reciprocation of the printing table 2, a transfer film 15 attached on the printing roll 5, an anilox roll 8 that transfers aligning solution to the transfer film 15, and a doctor roll 10 for uniformly coating the anilox roll 8 with the aligning solution. A substrate 13 is positioned on the printing table 2, and then the printing table 2 is moved toward the printing roll at a constant speed. When the printing roll 5 engages with the substrate 13, the printing roll rotates and the transfer film 15 attached on the printing roll 5 contacts the substrate 13 on the printing table 2 so that the aligning solution on the transfer film 15 is transferred to the substrate 13. Thus, an alignment layer (not shown) is formed on the substrate 13. The alignment layer is dried to remove moisture therein and then cured to be hardened. Next, the hardened alignment layer is rubbed by a rubbing fabric under constant pressure so that polymer chains in the alignment layer are arranged in a specified direction.
FIG. 3A is a plan view of a related art transfer film and FIG. 3B is an enlarged view of the region M of FIG. 3A. FIG. 4A is a cross-sectional view of the related art transfer film and FIG. 4B is an enlarged view of the region N of FIG. 4A. As shown in FIGS. 3A, 3B, 4A and 4B, the transfer film 15 includes convex portions 17 and a ground portion 19. Each convex portion 17 possesses the aligning solution and transfers the aligning solution to the substrate 13 of FIG. 2. The ground portion 19 surrounding the convex portions 17 does not contact the substrate 13 of FIG. 2. In general, the convex portion 17 has an average thickness T1 within a range of about 2.24 mm to about 2.84 mm, and the ground portion 19 has an average thickness T2 within a range of about 1.4 mm to about 2.0 mm.
The convex portion 17 includes a plurality of halftone dots 21, each of which has a shape of a truncated cone for containing the aligning solution of uniform quantity, and grooves 23, in which the aligning solution resides therein, are formed between the halftone dots 21. The plurality of halftone dots 21 has substantially the same height and the same size and spaces between the halftone dots 21 are also equal. That is, the grooves 23 have substantially the same depth and the same width. In addition, the halftone dots 21 of the convex portion 17 have sides inclined with respect to the lower surface of the transfer film 15. The convex portion 17 may includes the halftone dots of about 400 meshes.
FIG. 5A is a cross-sectional view of showing an alignment layer formed on a substrate according to the related art, and FIG. 5B is an enlarged view of the region E1 FIG. 5A. As shown in FIGS. 5A and 5B, an alignment layer 32 is coated on a substrate 30, which may be either an upper substrate or a lower substrate of a liquid crystal display device, and is then cured. The alignment layer 32 has a thickness within a range of about 700 Å to about 1,100 Å. However, the alignment layer 32, which is formed by the transfer film 15 illustrated in FIGS. 3A to 4B, may have a non-uniform thickness. That is, although the alignment layer 32 should have the thickness within a range of about 700 Å to about 1,100 Å, the alignment layer 32 may have a thickness within a range of about 1,400 Å to about 3,300 Å at edges thereof. Therefore, the alignment layer 32 at the edges has a thickness that is about two to about three times greater as compared with the central portion thereof. This problem causes a non-uniform gap between the color substrate and the array substrate in the liquid crystal display device. Thus, spots having different degrees of brightness occur in the displayed images when signal voltages are applied.
This problem of the alignment layer having a thicker thickness at the edges than at the central portion may be caused by a marginal phenomenon. The marginal phenomenon is as follows. In the printing apparatus of the alignment layer, the transfer film having halftone dots of the same size is attached on the printing roll, and the printing roll rotates with the attached transfer film while the table having the substrate thereon moves at a constant speed along with the surface of the transfer roll. As the convex portion of the transfer film including the halftone dots contacts the substrate, the aligning solution filled in the grooves of the convex portion of the transfer film is transferred to the substrate by the anilox roll. The aligning solution is gathered at the edges of the convex portion by the anilox roll as the printing roll rotates. Thus, the aligning solution is thicker at the grooves about leading and wayward edges of the convex portion. This phenomenon can be referred to as the marginal phenomenon.